World Housing Encyclopedia Report

Transcription

1 World Housing Encyclopedia Report Country: Nepal Housing Type: Traditional oval-shaped rural stone house. Contributors: Yogeshwar Krishna Parajuli Jitendra Kumar Bothara Bijay Kumar Upadhyay Primary Reviewer: Richard Sharpe Created on: 6/5/2002 Last Modified: 7/2/2003 This encyclopedia contains information contributed by various earthquake engineering professionals around the world. All opinions, findings, conclusions, and recommendations expressed herein are those of the various participants, and do not necessarily reflect the views of the Earthquake Engineering Research Institute, the International Association for Earthquake Engineering, the Engineering Information Foundation, John A. Martin & Associates, Inc. or the participants' organizations.

2 Table of Contents General Information...1 Architectural Features... 2 Socio-Economic Issues... 3 Structural Features... 5 Evaluation of Seismic Performance and Seismic Vulnerability... 9 Earthquake Damage Patterns Building Materials and Construction Process Construction Economics...14 Insurance...15 Seismic Strengthening Technologies References Contributors Figures...19

3 1 General Information 1.1 Country Nepal 1.3 Housing Type Traditional oval-shaped rural stone house. 1.4 Summary This is a typical rural construction concentrated in the central mid-mountains, particularly in Kaski, Syangja, Parbat, Baglung districts (the country is divided into 75 administrative districts). The basic function of these buildings is residential. These buildings are basically loose fit load bearing structures constructed of uncoursed rubble stone masonry walls and a timber structure for floor and roof. These buildings are owner-built and village artisans play a pivotal role. These buildings are expected to be extremely vulnerable in earthquake effects because of the loss of integrity during an event. FIGURE 1: Typical Building 1.5 Typical Period of Practice for Buildings of This Construction Type How long has this construction been practiced < 25 years < 50 years < 75 years < 100 years < 200 years > 200 years Is this construction still being practiced? Yes No 1.6 Region(s) Where Used This building type is concentrated in Kaski, Syangja, Parbat, and Baglung districts of Central Mid Mountains of the Western Development Region of Nepal (Nepal is divided into five development regions and seventy five districts which are further subdivided into small political units (56 municipalities and some 4000 Village Development Committees). The percentage of this building type in the total stock as well as total population inhibiting this building type is unknown. 1.7 Urban vs. Rural Construction Where is this construction commonly found? In urban areas In rural areas In suburban areas Both in rural and urban areas Additional Comments: These buildings are being gradually replaced by more modern building types even in rural areas. Page 1

4 2 Architectural Features 2.1 Openings Typically three to four openings are provided in each story, one for door and rest for windows in main building. Front façade has more openings than the back. Openings are limited in size. Openings constitute some 15-20% of total wall length. Spacing between openings is generally more than twice the length of opening. 2.2 Siting Is this type of construction typically found on flat terrain? Is this type of construction typically found on sloped terrain? (hilly areas) Is it typical for buildings of this type to have common walls with adjacent buildings? Yes No The typical separation distance between buildings is 10 meters 2.3 Building Configuration Building plan is oval in shape. 2.4 Building Function What is the main function for buildings of this type? Single family house Multiple housing units Mixed use (commercial ground floor, residential above) Other (explain below) 2.5 Means of Escape Buildings of this type haven't additional door besides the main entry. 2.6 Modification of Buildings There aren't modifications of bearing structures in these buildings usually. Page 2

5 3 Socio-Economic Issues 3.1 Patterns of Occupancy Single/ multiple families both live in a single house. 3.2 Number of Housing Units in a Building 1 units in each building. 3.3 Average Number of Inhabitants in a Building How many inhabitants reside in a typical building of this construction type? During the day / business hours During the evening / night < 5 5 to > 20 Other 3.4 Number of Bathrooms or Latrines per Housing Unit Number of Bathrooms: 0 Number of Latrines: 0 Additional Comments: This building type does not comprise attached toilet or bathroom. In the past, there were no latrines or bathrooms available in this type of house. Presently, toilets are constructed but away from the houses and in isolation. Insert additional text if applicable 3.5 Economic Level of Inhabitants Economic Status House Price/Annual Income (Ratio) Very poor / Poor / Middle Class / Rich / Additional Comments: A pricing system does not exist because of informal housing production mechanism. 3.6 Typical Sources of Financing What is the typical source of financing for buildings of this type? Owner Financed Personal Savings Informal Network: friends and relatives Small lending institutions/microfinance institutions Commercial banks / mortages Investment pools Combination (explain) Government-owned housing Other 3.7 Ownership Page 3

6 Type of Ownership/Occupancy Rent Own outright Own with Debt (mortgage or other) Units owned individually (condominium) Owned by group or pool Long-term lease Other Page 4

7 4 Structural Features 4.1 Lateral Load-Resisting System The load bearing walls carry the lateral loads. The masonry walls thus act as shear walls. The building has only a perimeter wall, which encloses the building space and also carries the loads. The roof and floor are loose fit timber structures, which act as flexible diaphragm and are not able to transfer the lateral load to wall piers according to their stiffness. 4.2 Gravity Load-Bearing Structure The gravity loads of the main building are carried by load bearing walls. Floor and roof are constructed of timber, which transfers their loads to the walls (typical thickness 450 mm mm), which carries the load to the foundation. These walls are carried by a strip foundation of uncoursed rubble stone masonry. The veranda (annex to the main building) is a lean-to structure to main building, which is supported by timber posts at one end. These posts are generally supported by an above-ground stone pedestal (no anchorage between stone and post). No rigid connection is made between column and beam being supported. Page 5

8 4.3 Type of Structural System Material Masonry Concrete Steel Timber Various Type of Load-Bearing Structure # Subtypes Stone masonry 1 Rubble stone (field stone) in mud/lime mortar or without walls mortar (usually with timber roof) 2 Massive stone masonry (in lime or cement mortar) Earthen walls 3 Mud walls 4 Mud walls with horizontal wood elements 5 Adobe block or brick walls 6 Rammed earth/pise construction Unreinforced brick masonry walls 7 Unreinforced brick masonry in mud or lime mortar 8 Unreinforced brick masonry in mud or lime mortar with vertical posts 9 Unreinforced brick masonry in cement or lime mortar (various floor/roof systems) Confined masonry 10 Confined brick/block masonry with concrete posts/tie columns and beams Concrete block masonry walls 11 Unreinforced in lime or cement mortar (various floor/roof systems) 12 Reinforced in cement mortar (various floor/roof systems) 13 Large concrete block walls with concrete floors and roofs Moment resisting 14 Designed for gravity loads only (predating seismic codes i.e. frame no seismic features) 15 Designed with seismic features (various ages) 16 Frame with unreinforced masonry infill walls 17 Flat slab structure 18 Precast frame structure 19 Frame with concrete shear walls-dual system 20 Precast prestressed frame with shear walls Shear wall structure 21 Walls cast in-situ 22 Precast wall panel structure Moment resisting frame 23 With brick masonry partitions 24 With cast in-situ concrete walls 25 With lightweight partitions Braced frame 26 Concentric 27 Eccentric Load-bearing 28 Thatch timber frame 29 Post and beam frame 30 Walls with bamboo/reed mesh and post (wattle and daub) 31 Wooden frame (with or without infill) 32 Stud wall frame with plywood/gypsum board sheathing 33 Wooden panel or log construction Seismic protection 34 Building protected with base isolation devices or seismic systems dampers Other 35 Page 6

9 4.4 Type of Foundation Type Description Shallow Foundation Wall or column embedded in soil, without footing Rubble stone (fieldstone) isolated footing Rubble stone (fieldstone) strip footing Reinforced concrete isolated footing Reinforced concrete strip footing Mat foundation No foundation Deep Foundation Reinforced concrete bearing piles Reinforced concrete skin friction piles Steel bearing piles Wood piles Steel skin friction piles Cast in place concrete piers Caissons Other 4.5 Type of Floor/Roof System Material Description of floor/roof system Floor Roof Masonry Vaulted Composite masonry and concrete joist Structural Concrete Solid slabs (cast in place or precast) Cast in place waffle slabs Cast in place flat slabs Precast joist system Precast hollow core slabs Precast beams with concrete topping Post-tensioned slabs Steel Composite steel deck with concrete slab Timber Rammed earth with ballast and concrete or plaster finishing Wood planks or beams with ballast and concrete or plaster finishing Thatched roof supported on wood purlins Wood single roof Wood planks or beams that support clay tiles Wood planks or beams that support slate, metal asbestos-cement or plastic corrugated sheets or tiles Wood plank, plywood or manufactured wood panels on joists supported by beams or walls Other Wood planks (or fire wood) and joists covered with thick mud overlay Additional Comments: Floor and roof structures are loose-fit elements, as if one component is stacked over the other (without any nailing). These therefore behave as flexible diaphragm. In past earthquakes such floors were just scattered due to shaking. 4.6 Typical Plan Dimensions Length: meters Width: meters Additional Comments: Length varies from 8 to 10 meters. Width varies from 6 to 8 meters. 4.7 Typical Number of Stories Typical Story Height 2.2 meters Additional Comments: Typical story height is meters. Page 7

10 4.9 Typical Span 1.7 meters Additional Comments: Span between the supports of floor and walls ranges from 1.5 to 2 meters usually. The building is oval shaped and there does not exist any internal walls for separating internal space, so the concept of span is not applicable Typical Wall Density Total wall density (total plan area of wall/ total plinth area) is around 25% General Applicability of Answers to Questions in Section 4 This is traditional structure of Nepal people. Page 8

11 5 Evaluation of Seismic Performance and Seismic Vulnerability 5.1 Structural and Architectural Features: Seismic Resistance Structural/ Architectural Feature Lateral load path Building configuration Roof construction Floor construction Foundation performance Wall and frame structuresredundancy Wall proportions Foundation- wall connection Wall-roof connections Wall openings Quality of building materials Quality of workmanship Maintenance Other 5.2 Seismic Features Statement True False N/A The structure contains a complete load path for seismic force effects from any horizontal direction that serves to transfer inertial forces form the building to the foundation. The building is regular with regards to both the plan and the elevation. The roof diaphragm is considered to be rigid and it is expected that the roof structure will maintain its integrity, i.e.. shape and form, during an earthquake of intensity expected in this area. The floor diaphragm(s) are considered to be rigid and it is expected that the floor structure(s) will maintain its integrity, during an earthquake of intensity expected in this area. There is no evidence of excessive foundation movement (e.g. settlement) that would affect the integrity or performance of the structure in an earthquake. The number of lines of walls or frames in each principal direction is greater than or equal to 2. Height-to-thickness ratio of the shear walls at each floor level is: 1) Less than 25 (concrete walls); 2)Less than 30 (reinforced masonry walls); 3) Less than 13 (unreinforced masonry walls). Vertical load-bearing elements (columns, walls) are attached to the foundations; concrete columns and walls are doweled into the foundation. Exterior walls are anchored for out-of-plane seismic effects at each diaphragm level with metal anchors or straps. The total width of door and window openings in a wall is: 1) for brick masonry construction in cement mortar: less than 1/2 of the distance between the adjacent cross walls; 2) for adobe masonry, stone masonry and brick masonry in mud mortar: less than 1/3 of the distance between the adjacent cross walls; 3) for precast concrete wall structures: less than 3/4 of the length of a perimeter wall. Quality of building materials is considered to be adequate per requirements of national codes and standards (an estimate). Quality of workmanship (based on visual inspection of few typical buildings) is considered to be good (per local construction standards). Buildings of this type are generally well maintained and there are no visible signs of deterioration of building elements (concrete, steel, timber). Structural Element Seismic Deficiency Earthquake-Resilient Features Earthquake Damage Patterns Wall - Binding material (mortar) for walling unit is too weak. - Walling units are irregular. - Absence of through stones. Frame (columns, beams) - Inadequate beam-to-column connection and beam-to-wall connection. - No anchorage between timber posts and foundation. Roof and floors - Flexible. - No interconnection between different structural elements. - No connection between walls and floor/ roof (in general). - Heavy floor Page 9

12 5.3 Seismic Vulnerability Rating Seismic Vulnerability Class 0 - probable value < - lower bound > - upper bound High (Very Poor Seismic Performance) Vulnerability Medium Low (Excellent Seismic Performace) A B C D E F 0 > Page 10

13 6 Earthquake Damage Patterns 6.1 Past Earthquakes Reported To Affect This Construction Year Earthquake Epicenter Richter magnitude(m) Maximum Intensity (Indicate Scale e.g. MMI, MSK) Additional Comments: No medium or major earthquakes observed in the area to date in known history (oral or written) so the performance of these buildings in a real earthquake is largely unknown. But buildings with similar construction materials and technology (but with different plan shape) have performed extremely poorly in past earthquakes. Page 11

14 7 Building Materials and Construction Process 7.1 Description of Building Materials Structural Element Building Material Characteristic Strength Mix Proportions/ Dimensions Comments Walls Rubble stone Not known/ Not relevant for strength Irregular boulders (size mm or less) Slates, lime stone, quartzite Foundations Mud Very low compressive strength and no tensile strength Used for mortar Frame Soft and hard wood Not known Depending on structural value Hard wood used for of the member members of high structural value (e.g. Columns, principal beams) where as softwood used for members with relatively low structural value (e.g.. Joists, purlins) Roof and floors Timber/ bamboo Not known Difficult to define because of selected use of multiple species 7.2 Does the builder typically live in this construction type, or is it more typically built by developers or for speculation? Yes, builders/ owners live in this construction type (house owner himself is part of construction team). 7.3 Construction Process The walls are constructed in a random uncoursed manner by using irregular stones bound with mud mortar. The stones are collected from quarries, riverbed or field, sometimes partially dressed. Space between interior and exterior wythes is filled with small stones and mud. The joists and rafters are just placed on walls without any anchorage or connection. These buildings are owner-built where village artisans play pivotal role. Simple tools such as chisels, hammers, saw etc are used for construction. 7.4 Design/Construction Expertise The artisans are without any formal training. The construction know-how is transferred from generation to generation or the people learn the process on site in a very informal way. The head mason is skilled but the level of know-how varies from person to person. No standard or minimum requirement exists for head or any other mason. The rest of the working team is composed of semi or unskilled personnel. 7.5 Building Codes and Standards Is this construction type addressed by codes/standards? Yes No Title of the code or standard: NBC203 : Guidelines for Earthquake Resistant Building Construction: Low Strength Masonry (Draft) 7.6 Role of Engineers and Architects Engineers /architects / technicians are not involved in this construction type. 7.7 Building Permits and Development Control Rules Page 12

15 Building permits are required Informal construction Construction authorized per development control rules Yes No Additional Comments: The building by-laws, building permit process and building construction controlling monitoring mechanisms only exists in municipalities and not in Village Development Committee (local authority at village level- rural areas). This is basically a rural house type where the building permit process does not exist. If this type of housing were to be constructed in a municipality, it would have to pass through the formal process (but the process does not require approval of structural drawings for this size of building). Present bylaws or regulation do not prohibit the construction of this type of building in municipal areas. 7.8 Phasing of Construction Construction takes place over time (incrementally) Building originally designed for its final constructed size Yes No 7.9 Building Maintenance Who typically maintains buildings of this type? Builder Owner(s) Renter(s) No one Other 7.10 Process for Building Code Enforcement There is no process for Building Code enforcement in rural areas (Village Development Committee areas) of Nepal Typical Problems Associated with this Type of Construction Weak construction materials/ and construction technology, lack of quality control, lack of earthquake resistant features are the problems associated with this type of construction. Again this type of construction does not match the modern use pattern of user. So it is being phased out with time. Page 13

16 8 Construction Economics 8.1 Unit Construction Cost (estimate) Cash flow in such construction is very minimal so it is difficult to price the building cost. 8.2 Labor Requirements (estimate) man-days (excluding effort required for collection of construction materials). Page 14

17 9 Insurance 9.1 Insurance Issues Earthquake insurance for this construction type is typically available Insurance premium discounts or higher coverages are available for seismically strengthened buildings or new buildings built to incorporate seismically resistant features Yes No 9.2 If earthquake insurance is available, what does this insurance typically cover/cost? Not applicable. Page 15

18 10 Seismic Strengthening Technologies 10.1 Description of Seismic Strengthening Provisions Type of intervention Structural Deficiency Description of seismic strengthening provision used Retrofit Roof/ floor Enhancement of integrity, anchorage with walls, bracing (Strengthening) Walls Insertion of bond stones, bandages at different levels, splint at critical sections Timber Frame Bracing of frame (knee bracing, diagonal bracing) to strengthen beam-column connection, anchorage of column to foundation New Construction Roof/ floor Enhancement of integrity, anchorage with walls, bracing Walls Use of cement mortar, use of bond stones, bands at different levels, vertical bars at critical sections Timber frame Knee or diagonal bracing of beam-column joints, connection of column to foundation 10.2 Has seismic strengthening described in the above table been performed in design practice, and if so, to what extent? 10.3 Was the work done as a mitigation effort on an undamaged building, or as repair following earthquake damage? 10.4 Was the construction inspected in the same manner as new construction? 10.5 Who performed the construction: a contractor, or owner/user? Was an architect or engineer involved? 10.6 What has been the performance of retrofitted buildings of this type in subsequent earthquakes? Page 16

19 11 References 1994, Appendix-A: Prototype Building inventory; the Development of Alternative Building Materials and Technologies for Nepal, UNDP/UNCHS (Habitat) Sub-project Nep 88/054/21.03, His Majesty's Government of Nepal, Ministry of House and Physical Planning. 1994, NBC 203 Guidelines for Earthquake Resistant Building Construction: Low Strength Masonry, UNDP/UNCHS (Habitat) Sub-project Nep 88/054/21.03, His Majesty's Government of Nepal, Ministry of House and Physical Planning. Page 17

20 12 Contributors Name Yogeshwar Krishna Parajuli Jitendra Kumar Bothara Bijay Kumar Upadhyay Title Architect Structural Engineer Building Technologist Affiliation National Team Leader, Nepal Team Member, Nepal Team Member, Nepal National Building Code Development Project; C/O TAEC Consult P. Ltd. National Building Code Development Project; C/O TAEC Consult P. Ltd. National Building Code Development Project; C/O TAEC Consult P. Ltd. Address Shankhamul Shankhamul City Kathmandu Shankhamul, Kathmandu Kathmandu Zipcode Country Nepal Nepal Nepal Phone Fax Webpage Page 18

21 13 Figures FIGURE 1: Typical Building Page 19

22 FIGURE 2: Key Load-Bearing Elements Page 20

23 Page 21

24 FIGURE 3: Plan of a Typical Building Page 22

25 Page 23

26 FIGURE 4: Critical Structural Details FIGURE 5: An Illustration of Key Seismic Features and/or Deficiencies Page 24

27 FIGURE 6: Illustration of Seismic Strengthening Techniques Page 25